Large-scale power cuts are rare in Hong Kong.
Electricity supplier CLP Power said one of its cable bridges in the northern district Yuen Long had caught fire on Tuesday at 7 p.m., cutting power to around 160,000 customers. There had been no injuries, it said.
Power to essential services, including hospitals and railways, had been restored but two more days would be needed to restore supply to a remaining 20,000 customers, the company said in a statement.
The government said: "We understand that the power outage incident in this very hot weather has greatly affected the public, and we are deeply sorry for this." Hong Kong's forecast maximum temperature for Wednesday and Thursday was 31 degrees, with high humidity.
The government added in its statement that it had asked CLP Power to determine the cause of the accident and submit a report.
The education bureau said dozens of schools in the area had lost power and would need to suspend classes on Wednesday.
(Reporting by Farah Master; Editing by Bradley Perrett)